Methane (CH4) is a greenhouse gas (GHG) produced primarily by methanogenic microbes that are found in natural ecosystems (e.g. wetlands, oceans and lakes) and the gastrointestinal tract of invertebrates and vertebrates, such as termites and ruminants. Every year ˜429-507 Tg of CH4 are removed from the atmosphere and ˜40 Tg from the stratosphere through reactions with hydroxyl (OH) radicals; and ˜30 Tg by CH4-oxidizing bacteria in soil.
Nevertheless, anthropogenic GHG emissions have been increasing rapidly, with the CH4 concentration in the atmosphere now more than twofold higher than in the early 1800s. Methane is very effective in absorbing solar infrared radiation and has a global warming potential 25 times greater than CO2. Consequently, its accumulation in the atmosphere contributes considerably to climate change. One of the main sources of anthropogenic CH4 can be attributed to agricultural activities, including ruminant livestock.
According to a recent UN report, cattle-rearing generates more global warming greenhouse gases, as measured in CO2 equivalent, than transportation. In Australia, ruminants are estimated to contribute ˜10% of the total GHG emissions. Ruminants produce CH4 as a by-product of the anaerobic microbial fermentation of feeds in the rumen and, to a lesser extent, in the large intestine. The ruminal microbial community is highly diverse and composed of bacteria, protozoa, fungi, and bacteriophages that act collectively to ferment ingested organic matter (OM), resulting in CO2, H2, volatile fatty acids (VFAs), and formates. Methanogenic archaea present in the rumen use these end-products and produce CH4. Although the production of CH4 reduces the partial pressure of H2, which could otherwise inhibit rumen fermentation, it also reduces the amount of energy and carbon available for formation of VFAs essential for ruminant nutrition. Most of the CH4 produced in ruminants is exhaled and belched by the animal and represents a loss of up to 12% of gross energy intake.
Mitigation strategies that reduce enteric CH4 formation are important, and methods of reducing total gas production and/or methane production in ruminant animals represent a major challenge.